The present embodiment relates generally to cement compositions and methods for cementing an area using such cement compositions. In certain examples, methods for cementing in a subterranean zone are described.
In the drilling and completion of an oil or gas well, a cement composition is often introduced in the wellbore for cementing pipe string or casing. When the desired drilling depth of the well is reached, a cement composition is pumped into the annular space between the walls of the wellbore and the casing. The cement composition sets in the annular space, supporting and positioning the casing, and forming a substantially impermeable barrier, or cement sheath, which isolates the wellbore from subterranean zones.
Most cements contain chromium compounds because the raw material (typically shale) from which cements are made usually contains chromium compounds. When the cements are mixed with water, the chromium compounds appear as Cr+6 (chromate) dissolved in the water. The dissolved chromate presents a health risk to humans, for example, allergic reactions.
Ferrous sulfate is known to reduce Cr+6 to Cr+3, which is less soluble in cement-water mixtures. Ferrous sulfate has been dry blended with cement during the manufacturing of the cement, but in such a blend, the ferrous sulfate remains effective for only a limited period of time. In addition, the solubility of ferrous sulfate is such that it will settle out of solution if formulated as a liquid additive.